How do you know they're not sentient? That they're property is irrelevant: we see from example that slaves are kept in the Star Wars universe.exodus said:
Not true, R2-D2 & C-3PO believe they are sentient but aren't, they're just programmed to mimic human behavior. Droids are built to be property, therefore aren't sentient beings.
Frankly, the Star Wars saga makes a rather good bit more sense if you suppose that Artoo is the supreme Jedi master.